As the developing world continues to urbanize, and Africa and Asia approach the point at which they will become more urban than rural, new investments in demographic data and research methods will be required to understand and address the public health and socioeconomic needs of urban residents. At a minimum, policy-makers will need defensible estimates of city population sizes, rates of growth, and geographic extents if they are to formulate effective development strategies. At present the demographic research community cannot supply such estimates (National Research Council, 2003) as it relies on out-of-date methods and sources of data. Yet over the two past decades, a wealth of new data, largely from surveys, has become available for developing countries, with considerable potential to improve city estimates and projections. Further, recent methodological advances have suggested new ways by which satellite imagery can be used to detect urban areas and measure their spatial extents (Balk et al, 2005, Small 2005, Elvidge et al., 1997). Far more spatially-coded data-such satellite sources among them-are available now than was the case 25 years ago. Moreover, urban data from demographic surveys have greatly strengthened the basis for estimating the demographic components of urban growth and will continue to do so. Having taken note of these positive developments, we are now posed to not only to substantially improve our estimates and projections of city growth, but also to establish an explicitly spatial basis for its city estimates and forecasts. The goal of the proposed research team-including scientists from the Columbia University's Center for International Earth Science Information Network (CIESIN) and Lamont-Doherty Earth Observatory (LDEO), the Population Council, and the UN Population Division-is to produce and evaluate new spatially-explicit demographic methods for estimating and projecting city populations in the developing world. The work will draw on recent techniques in demography, economics, and remote-sensing. Over the next few decades, Asia and Africa will likely cross an historic threshold, becoming, for the first time, more urban than rural. The proposed work is systematic improvement in the estimation and forecasting of the future urban world, and will offer much greater clarity on where, when, and the pace at which those transitions may occur and the health and welfare implications thereof.